It is simple science... but there is no such thing as a "transgender". Gender is based on chromosomes, not a state of mind. No amount of surgery, hormone therapy or cross-dressing will change gender. No one can change gender... ever. A man can become a transvestite, but never change gender.
That is the science of it.
Maybe a little picky, but words have genders, people and animals have sexes. There’s no way to be a transgender. You can be a transsexual, I suppose, but I guess that word has gone out of political favor.